Effects of Tungsten Carbide Thermal Spray Coating by Hvof on Fatigue and Corrosion

.Surface and Coatings Technology 138 2001 113124

Effects of tungsten carbide thermal spray coating byHPHVOF and hard chromium electroplating on AISI 4340

high strength steel

Marcelino P. Nascimentoa,, Renato C. Souzab, Ivancy M. Miguela, Walter L.Pigatinc, Herman J.C. Voorwalda

aState Uniersity of Sao Paulo- DMT-UNESPFEG, A. Ariberto Pereira da Cunha, 333-GuaratinguetaSPBR-CEP: 12500-000, Brazil

bFAENQUILDEMAR, LorenaSPBR-CEP:12600-000, BrazilcEMBRAER-LIEBHERREDE, Sao Jose dos CamposSPBR-CEP: 12237-540, Brazil

Received 26 August 1999; received in revised form 5 October 2000; accepted 7 November 2000

Abstract

In cases of decorative and functional applications, chromium results in protection against wear and corrosion combined withchemical resistance and good lubricity. However, pressure to identify alternatives or to improve conventional chromiumelectroplating mechanical characteristics has increased in recent years, related to the reduction in the fatigue strength of the basematerial and to environmental requirements. The high efficiency and fluoride-free hard chromium electroplating is animprovement to the conventional process, considering chemical and physical final properties. One of the most interesting,environmentally safer and cleaner alternatives for the replacement of hard chrome plating is tungsten carbide thermal spray

.coating, applied by the high velocity oxy-fuel HVOF process. The aim of this study was to analyse the effects of the tungstencarbide thermal spray coating applied by the HPHVOF process and of the high efficiency and fluoride-free hard chromium

.electroplating in the present paper called accelerated , in comparison to the conventional hard chromium electroplating on theAISI 4340 high strength steel behaviour in fatigue, corrosion, and abrasive wear tests. The results showed that the coatings weredamaging to the AISI 4340 steel behaviour when submitted to fatigue testing, with the tungsten carbide thermal spray coatingsshowing the better performance. Experimental data from abrasive wear tests were conclusive, indicating better results from theWC coating. Regarding corrosion by salt spray test, both coatings were completely corroded after 72 h exposure. Scanning

.electron microscopy technique SEM and optical microscopy were used to observe crack origin sites, thickness and adhesion inall the coatings and microcrack density in hard chromium electroplatings, to aid in the results analysis. 2001 Elsevier ScienceB.V. All rights reserved.

Keywords: Tungsten carbide thermal spray coating; Hard chromium electroplating; Abrasive wear; Corrosion; Fatigue; HPHVOF

Corresponding author. Tel.: 55-12-525-2800; fax: 55-12-515-2466. .E-mail address: [email protected] M.P. Nascimento .

0257-897201$ - see front matter 2001 Elsevier Science B.V. All rights reserved. .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 1 1 4 8 - 8

( )M.P. Nascimento et al.Surface and Coatings Technology 138 2001 113124114

1. Introduction

Chromium plating is the most used electrodepositedcoating to obtain high levels of hardness, resistance towear and corrosion and a low coefficient of friction forapplications in the aerospace, automotive and

petrochemical fields 1,2 . Chromium plating proper-ties, such as hardness and microcrack density, changewith the bath composition, current density, bath agita-

tion, temperature, etc. 3,4 . Among other things, asignificant characteristic of chromium electroplating isthe high tensile residual internal stresses originatingfrom the decomposition of chromium hydrides during

the electrodeposition process 35 . These high tensilestresses in electroplated chromium coatings increase asthickness increases and are relieved by local micro-cracking during electroplating. Therefore, basically, mi-crocrack density is related to the high tensile residualinternal stresses, hardness, and corrosion resistance 2,3,6 . It was observed that the residual stressesthrough-thickness decrease with the depth of the coat-ing and increase again at the coatingsubstrate inter-

face 7 . Bending fatigue tests on samples with differentcoatings and coating conditions indicate that the fa-tigue strength is dependent on the fracture behaviourof the substrates and on the hardness and residualstresses at the substrate surface. It was also observedthat the hard chromium electroplating reduces the

fatigue strength of a component 8 . Due to this fact,the design of hard chromium plated components, whichare subjected to dynamic loads, may consider this nega-tive influence to guarantee safety during operation.Therefore, the use of effective methods to improve thefatigue strength shall be considered. Shot peening is awell-known process to increase fatigue life of structuressubjected to constant and variable amplitude loading.The compressive residual stress obtained by surfaceplastic deformation is responsible for the increase infatigue strength in shot peened mechanical compo-

nents 9 . Compressive residual stresses induced bymachining processes are also responsible for the im-

provement in fatigue resistance of AISI 4340 steel 10 .Increase in the fatigue crack propagation resistance inAISI 4340 steel with electron beam surface hardeningwas associated to residual stress distribution and mi-

crostructural characteristics 11 . However, problemsconcerning chrome plating, such as health and environ-mental hazards, increasing costs and a performance notin accordance with the specifications, have resulted in a

search to identify possible alternatives 12 . Aircraftlanding gear manufacturers are considering tungsten

.carbide WC thermal spray coatings applied by the .high velocity oxy-fuel HVOF process as an alternative

to hard chrome plating. The question to be answered isif the performance of the alternative candidate is at

least comparable to results obtained for hard chromeplating. Comparisons of experimental data showed bet-ter corrosion resistance for several HVOF coatingswith respect to chrome plating. In the case of fatigueand friction tests, the results were acceptable, indicat-ing interesting perspectives on the use of tungsten

carbide coating to replace chrome-plating 1 . Analysisof the wear performance of tungsten carbide coatedsamples in the presence of air, aqueous and aqueousabrasive media indicated better results in terms ofvolume loss and change in surface roughness than for

the mild steel substrate 13 . The objective of thisresearch is to compare the influence of the tungstencarbide thermal spray coating applied by HPHVOFand hard-chromium plating on the fatigue strength,abrasive wear and corrosion resistance of AISI 4340steel. SN curves were obtained in rotating bendingand axial fatigue tests for the base material, chromiumplated, and tungsten carbide coated specimens.

2. Experimental procedures

AISI 4340 steel is widely used in aircraft componentswhere strength and toughness are fundamental designrequirements. The chemical analysis of the materialused in this research indicates accordance with specifi-cations.

The fatigue experimental program was performed onrotating bending and axial fatigue test specimens ma-chined from hot rolled, quenched and tempered barsaccording to Figs. 1 and 2, respectively. The specimenswere polished in the reduced section with 600 gritpapers, inspected dimensionally and by magnetic parti-cle inspection. Fatigue tests specimens were quenched

.from 815845C in oil 20C and tempered in therange of 5205C for 2 h. The mechanical propertiesof the material after the heat treatment are: hardnessof 39 HRC; yield tensile strength of 1118 MPa, andultimate tensile strength of 1210 MPa. After finalpreparation, samples were subjected to a stress relieveheat treatment at 190C for 4 h to reduce residualstresses induced by machining. Average superficialroughness in the reduced section of the samples was

.R 2.75 m and a standard deviation S.D. of 0.89am.

Rotating bending fatigue tests were conducted usinga sinusoidal load of frequency 50 Hz, and load ratioR1, at room temperature. For axial fatigue tests, asinusoidal load of frequency 50 Hz and load ratioR0.1 was applied throughout this study. Both testsconsider as fatigue strength the complete fracture ofthe specimens or 107 load cycles. Four groups of fa-tigue specimens were prepared to obtain SN curvesfor rotating bending fatigue and axial fatigue tests.

( )M.P. Nascimento et al.Surface and Coatings Technology 138 2001 113124 115

Fig. 1. Rotating bending fatigue testing specimen.

2.1. For rotating bending fatigue tests

12 smooth samples of base material; 13 samples with 160 m thickness of conventional

hard chromium electroplated; 13 samples with 100 m thickness of accelerated

hard chromium electroplated; and 13 samples with 100 m thickness of tungsten car-

bide thermal spray coated by HPHVOF process.

2.2. For axial fatigue tests

10 smooth samples of base material; 15 samples with 160 m thickness of conventional

hard chromium electroplated; 7 samples with 100 m thickness of accelerated

hard chromium electroplated; and 13 samples with 100 m thickness of tungsten car-

bide thermal spray coated by HPHVOF process.

The tungsten carbide thermal spray coated speci-mens were blasted with aluminium oxide mesh 90 toenhance adhesion. To compare experimental data, six

Fig. 2. Axial fatigue testing specimen.

rotating bending fatigue specimens were tested withoutblasting.

2.3. Salt spray test

The performance of the coatings was evaluated withrespect to chemical corrosion in specific environment.The samples were prepared from normalised AISI 4340steel with 1 mm thickness, and 76 mm width and 254mm length, surface roughness R 0.2 m, and in theafollowing conditions:

accelerated hard chromium electroplated16, 36and 49 m thickness;

conventional hard chromium electroplated16, 36and 49 m thickness; and

tungsten carbide thermal spray coated100 mthickness.

Experimental tests were conducted in accordancewith ASTM B 117, in 5 wt.% NaCl, pH of 6.57.2, at35C. The samples were supported at 20 from thevertical. The results were analysed by Image Pro Plussoftware.

2.4. Abrasie wear test

The performance of the coating was also evaluatedwith respect to abrasive wear. For abrasive wear tests,samples were prepared from annealed AISI 4340 steelwith 4 mm thickness and 100 mm square, according toFED-STD-141C. The samples were divided in threegroups; two coated with 100-m thickness of acceler-ated and conventional hard chromium electroplating,respectively, and one group coated with 100-m thick-ness of tungsten carbide coating. The wear tests wereconducted with a Taber abraser, at room temperature,using a 10-N load and CS-17 abrading wheel for hardchromium electroplating and diamond wheel for tung-sten carbide coating. The results were analysed by wear

. index mg1000 cycles and total wear mg10 000 cy-.cles data.


2.5. Tungsten carbide coating

The tungsten carbide thermal spray coating appliedby HPHVOF system, used WC powder with 12% Co,resulting in thickness equal to 100 m. The averagesuperficial roughness in the reduced section of thesamples was R 4 m and a S.D. of 0.39 m, in theaas-deposited condition.

2.6. Hard chromium electroplating

The conventional hard chromium electroplating wascarried out from a chromic acid solution with 250 glof CrO and 2.5 gl of H SO , at 5055C, with a3 2 4current density from 31 to 46 Adm2, and a speed ofdeposition equal to 25 mh. A bath with a singlecatalyst based on sulfate was used.

The accelerated hard chromium electroplating wascarried out from a chromic acid solution with 250 glof CrO and 2.7 gl of H SO , at 5560C, with a3 2 4current density from 55 to 65 Adm2, and a speed ofdeposition equal to 80 mh. A bath with a doublecatalyst, one based on sulfate and the other withoutfluoride, was used. After the coating deposition, thesamples were subjected to a hydrogen embrittlementrelief treatment at 190C for 8 h. The average surfaceroughness of the hard chromium electroplating wasR 3.13 m in the reduced section and a S.D. of 0.79am in the as-electroplated condition.

For the microcrack determination in both hardchromium electroplating, samples were prepared from

.normalised AISI 4340 steel R 0.2 m , 1 mm thick-aness, 25 mm width and length, and with acceleratedand conventional hard chromium electroplating both

with 100 m thickness, which resulted in a surfaceroughness of R 0.74 m for the former and R 1.6a am for the later, in the as-electroplated condition. Thesurface microcracks were enhanced through anodicetching for 30 s with a current density equal to 25Adm2 in the same chromium bath and later analysedusing an optical microscope model NikonApophot.All surface roughness data measured in this researchwas obtained by Mitutoyo 301 equipment using a cut-offof 0.8 mm.

The analysis of fracture surface was carried out onrotating bending fatigue specimens by scanning elec-tron microscope, model LEO 435 vpi and Zeiss DSM950. The metallographic analysis was carried out onoptical microscope model Neophot 21.

3. Results and discussion

3.1. Fatigue test

The SN curves for the rotating bending and axialfatigue tests for the base metal and coated specimensare presented in Figs. 3 and 4, respectively.

Fig. 3 shows that the effect of coating in the rotatingbending fatigue test is to decrease the fatigue strengthof AISI 4340 steel. The tendency is observed for low

4. 5.number of cycles 10 , high number of cycles 10 andfor the fatigue limit, 107 cycles, and is represented inTable 1. One sees that the specimens coated withtungsten carbide applied by the HVOF process show alower decrease in fatigue strength. This may be at-tributed to the process itself. It is well known thatHVOF thermal spray process produces compressive

Fig. 3. SN curves for rotating bending fatigue tests.


Fig. 4. SN curves for axial fatigue tests.

residual internal stresses within the substrate, whichare formed from mechanical deformation on the sur-face during particle impact. This is confirmed by thethrough-thickness residual stress behaviour shown inFig. 5. These surface deformations counteract the ten-

sile shrinkage stresses of the coating caused by fastcooling and solidification as particles strike the surface.These tensile stresses in the coating also generatecompressive stresses within the surface of the subs-trate. However, there was a reduction in the fatigue

Table 1Rotating bending fatigue strength.

Rotating bending fatigue strength4 5 7 . . .Group Low cycles 10 High cycles 10 limit 10

. . .Base material 950 MPa 85% 700 MPa 63% 615 MPa 59%ys ys ys . . . .Treat. T. carb 100 m 900 MPa 80% 610 MPa 54% 531 MPa 47.5%ys ys ys . . . .Tungsten carb. 100 m 900 MPa 80% 570 MPa 51% 531 MPa 47.5%ys ys ys

. . . .Conv. chrome 160 m 840 MPa 75% 500 MPa 45% 321 MPa 29%ys ys ys . . . .Accel. chrome 100 m 730 MPa 65% 340 MPa 30% 280 MPa 25%ys ys ys

Fig. 5. Through-thickness residual stress distribution for WC HPHVOF thermal spray coating.


.Fig. 6. Microcracks surface network in conventional a and acceler- .ated b hard chromium electroplating. Anodic etching from 252 .Adm for 30 s 200 .

strength of AISI 4340, despite the compressive residualstresses induced by the process. This can be due to thehigh density of pores and oxide inclusions in the coat-ing that commonly form during the process. Thermalspray is generally conducted in air so chemical interac-tions occur, notably oxidation, which can be evident inthe coating microstructure as oxide inclusions, mainly

in grain boundaries 14 . These inclusions in coatingsubsurfaces are possible cracks or nucleationinitiationsites. From Fig. 3, it is possible to observe that thealuminium oxide blasting is responsible for a smallimprovement in the fatigue strength. Considering bothhard chromium electroplated rotating bending fatigueresults, one sees the negative influence of coating onthe fatigue strength of the steel. From the analysis ofthese two coatings, it is possible to observe the betterperformance of the conventional hard chromium inrelation to the accelerated hard chromium electroplat-ing, despite the higher thickness of the former. Thismay be attributed to the lower microcrack density ofthe conventional hard chromium electroplating in com-parison with the accelerated hard chromium electro-plating as showed in Fig. 6. The microcracks densityquantitative analysis indicated median values of 1512

microcrackscm and a S.D. of 190.6 microcrackscmfor the accelerated hard chromium electroplating, and223 microcrackscm with standard deviation of 57.5microcrackscm for the conventional hard chromiumelectroplating.

Microcracks form when the high tensile residualinternal stresses exceed the cohesive strength of thechromium deposits and affect the fatigue behaviour ofa plated part. Therefore, microcrack density arises as arelief of the tensile residual internal stresses, whichincrease when the chromium thickness increases. Pina

et al. 7 showed that the microcrack density changesalong the thickness, being higher at the core and lowerat the surface of the coating and in the substratecoat-ing interface due to the balance between the residualstresses. On the surface of the coating, the microcracksarise in a network shape, without preferential directionand characterising an equi-biaxial residual stress state.With respect to residual stresses, an inverse behaviourfrom that observed for the microcrack occurred. There-fore, in general, the higher the microcrack density, thehigher the tensile residual internal stresses andortheir relief. This means that the accelerated hardchromium electroplating is responsible for higher ten-sile residual internal stresses andor present the high-est crack initiationpropagation front amount. How-ever, the different microcrack densities between bothhard chromium electroplatings practically produced thesame effect in low cycle fatigue, since crack growthoccurs after few cycles of fatigue testing. In general,the fatigue strength for all conditions studied was re-duced due to the high tensile residual internal stresses,microcrack density, and high adhesion at thecoatingsubstrate interface, which allows the crack togrow from the coating through the interface into thebase metal.

Fig. 5 shows the residual internal stress profile fortungsten carbide thermal spray coating. From experi-mental points, one sees that the residual internalstresses change throughout coating thickness, from 100MPa tensile stress near to the surface, reaching 350MPa maximum tensile stress at a 0.025-mm depth anddecreasing to the maximum compressive stress of 680MPa at a 0.07-mm depth, increasing again into the basemetal, becoming tensile stress at a 0.20-mm depth. Thecurve of through-thickness residual stress was plottedbased on three specimens and obtained by the modifiedlayer-removal method for thermal spray coating and

substrates 18,19 . The through-thickness residualstresses change from approximately 300 MPa tensile at0.025-mm depth to approximately 680 MPa, compres-sive at 0.06 mm from the surface. This means that thecrack initiation may occur easily on the coating surface,but its propagation throughout the thickness may bedelayed when the compressive residual stress site isreached.


Table 2Axial fatigue strength

Axial fatigue strength4 a 5 7 . . .Group Low cycles 10 High cycles 10 Limit 10

. . .Base material 1330 MPa 119% 1125 MPa 101% 850 MPa 76%ys ys ys . . . .Tungsten carb. 100 m 1350 MPa 121% 850 MPa 76% 750 MPa 67%ys ys ys

. . . .Conv. chrome 160 m 1200 MPa 107% 650 MPa 58% 400 MPa 36%ys ys ys . . . .Accel. chrome 100 m 1150 MPa 103% 650 MPa 58% 400 MPa 36%ys ys ys

a 4 . Projection of the curves until the respective number of cycles 10 .

For hard chromium electroplating residual stresses, Pina et al. 7 showed that, despite the fact that micro-

cracks result in residual stress relief in the coating, thestresses still remain high at the surface approx. 800

. MPa , decreasing in direction to the core approx. 200.to 300 MPa , and increasing again at the interface to

values which depend on the substrate material.Fig. 4 shows the axial fatigue testing results, indicat-

ing the decrease in fatigue strength for all specimenscoated with tungsten carbide thermal spray and hardchromium electroplating, in comparison to the basematerial. Comparing the curves, one sees the negativeinfluence of coatings on the fatigue strength of thesteel, with the same tendency observed previously inthe rotating bending fatigue tests. This behaviour canalso be explained by high tensile residual internalstresses, oxide inclusions, pores and microcracks inher-ent from each process. Table 2 indicates the axialfatigue strength tendency of all specimens groups, basedon Fig. 4. The better performance of tungsten carbidecoated specimens in comparison to the hard chromiumplating may also be attributed to the lower tensileresidual internal stresses in the coating of the former,compressive residual stresses on the substrate surfaceand in the subsurface due to the particles impact effect,as well as by interactions between surfacesubstrateresidual stresses. Note in the axial fatigue test that thedifferent microcrack density between both hardchromium electroplatings did not play an importantrole in the specimens performance, as occurred in

rotating bending fatigue tests. A comparison of Tables1 and 2, as shown in Table 3, indicates the higherfatigue strength shown in axial fatigue test in relationto the rotating bending fatigue tests. In addition to thelower specimen dimensions, this is in accordance withthe fact that the rotating bending fatigue tests aremore severe as a result of the effect of the bendingmoment which increases the tensile stresses on surfacefrom where, in general, the fatigue cracks grow. How-ever, Table 3 shows a higher decrease in the fatiguestrength as a function of the number of cycles incomparison to the rotating bending fatigue test results,for each level of stress. This may also be due to radialthroughout thickness crack propagation to the basemetal, in a direction normal to the maximum tensilestress and resulting in lower fatigue life of the speci-

men 15 .Fig. 7 shows a typical fracture surface from the base

metal, indicating that the fatigue crack nucleationstarted at the surface. In Fig. 8, several crack frontsthat may be associated to the microcracks density origi-nated from the plating process are represented.

From Fig. 9, which represents fracture surface froma rotating bending fatigue specimen electroplated withaccelerated hard chromium and tested at 871 MPa, onesees the coating homogeneity, strong adhesion subs-tratecoating, and microcracks distributed along thick-ness in a radial shape.

Fig. 10 shows a micrograph of a tungsten carbidethermal spray coated specimen blasted with aluminium

Table 3Fatigue strength in number of cycles in the rotating bending and axial fatigue tests

Fatigue testinga a . .Stress Rotating. bending fatigue data % Axial fatigue data %

Base mat. W.C. C.H.C. A.H.C. Base mat. W.C. C.H.C. A.H.C. .MPa Cycles Cycles Cycles Cycles Cycles Cycles Cycles Cycles

7 5 . . . . . .850 22 000 12 000 55 9500 43 6000 27 10 10 1 30 000 0.30 30 000 0.307 6 . . . . . .750 60 000 24 000 40 18 000 36 9000 18 10 10 10 42 000 0.42 42 000 0.427 7 . . . . . .650 299 000 56 000 19 36 500 12 14 200 5 10 10 100 60 000 0.60 60 000 0.60

a Values contained in parenthesis are the rate between the number of cycles of the coated material and the number of cycles of basemetal, in percentage.


Fig. 7. Typical fracture surface from base metal. Rotating bendingfatigue test.

oxide. To compare, Fig. 11 indicates a coating profilewithout the blasting treatment. It is possible to observethat the aluminium oxide blasting is responsible for theincrease in roughness at the substratecoating inter-face, with a consequent improvement in adhesion.

The important characteristics of the hard chromiumelectroplating are the homogeneity of the coatings and

Fig. 8. Fracture surface from 100 m of accelerated hard chromiumelectroplating.

the excellent adhesion with the base metal, representedin Fig. 12. From Figs. 1012, it is also possible toobserve that, in both cases, the base metal microstruc-ture was not affected by the deposition process.

.Fig. 9. Fracture surface of samples accelerated hard chromium electroplated and tested at 871 MPa 500 .


Fig. 10. Tungsten carbide coating aluminium blasted specimen 200. .

3.2. Abrasie wears tests

The abrasive wear resistance of HPHVOF WCcoating and hard chromium plating was evaluated, andthe results in terms of wear weight loss are representedin Table 4 and Fig. 13. Comparing the abrasive wearresistance, one sees the better performance of samplescoated with WC, with lower wear weight loss than thehard chromium electroplated specimens. This may beattributed to the higher hardness and oxide contentinto the tungsten carbide coating. Coatings of highoxide content are usually harder and more wear resis-

tant 14 . However, initially, in the first 1000 cycles, analmost equivalent value of wear weight loss wasobserved for the tungsten carbide coating and conven-tional hard chromium electroplating, and a higher wearweight loss for the accelerated hard chromium. Thishigher hardness of tungsten carbide coating, whichshould result in lower wear weight loss in comparisonwith the conventional hard chromium electroplating,was damaged due to its higher surface roughness. Withrespect to both hard chromium electroplating, in thesubsequent cycles, the wear weight loss of the acceler-ated hard chromium electroplating decreases with theincrease in number of cycles in a parabolic way, result-ing after 10 000 cycles in lower wear weight loss thanthe conventional hard chromium plating. This may beexplained by the through-thickness hardness variation,in accordance with Table 5, in which the microhardnessdata of tungsten carbide coating and conventional hardchromium electroplating is also indicated. The lower

.Fig. 11. Tungsten carbide coating specimen 100 .

hardness on the accelerated hard chromium electro-plating surface and its increase through-thickness mayexplain the decrease in the wear weight loss after anumber of cycles. Theoretical calculations of the re-spective wear depth caused by abrasive wheels after10 000 cycles were 38.0 and 40.8 m for the acceleratedand conventional hard chromium electroplating, re-spectively, and 9.50 m for the tungsten carbide ther-mal spray coating.

Therefore, for both hard chromium electroplatingsthe wear depth values are associated to the hardnesschange site through-thickness. This may also be associ-ated to the higher microcrack density and higher hard-ness in the accelerated hard chromium electroplating,

.Fig. 12. Hard chromium electroplated specimen 100 .


Table 4Abrasive wears weight loss

.Abrasive wear Taber abraser

Cycles Tungsten carbide Accelerated hard chromium Conventional hard chromium .N . . .Total mg mg1000 Depth m Total mg mg1000 Depth m Total mg mg1000 Depth m

1000 2.20 2.20 1.57 8.30 8.30 11.67 2.83 2.83 3.972000 3.90 1.70 2.78 13.40 5.10 18.81 5.57 2.74 7.823000 4.70 0.80 3.56 16.50 3.10 23.16 8.33 2.76 11.704000 7.97 3.27 5.69 19.10 2.60 26.81 11.33 3.00 15.905000 10.80 2.83 7.72 21.00 1.90 29.47 14.83 3.50 20.816000 13.03 2.23 9.31 22.60 1.60 31.72 17.63 2.80 24.747000 14.20 1.17 10.15 24.10 1.50 33.82 20.60 2.97 28.918000 15.80 1.60 11.29 25.30 1.20 35.51 22.93 2.33 32.209000 17.40 1.60 12.43 26.30 1.00 36.91 25.83 2.90 36.25

10 000 18.87 1.47 13.48 27.10 0.80 38.80 29.13 3.30 40.80

Median 1.89 mg1000 cycles 2.71 mg1,000 cycles 2.91 mg1000 cyclesStandard 0.75 mg1000 cycles 2.34 mg1000 cycles 0.88 mg1000 cyclesdeviation

and so in the higher amount of edges, resulting inlower fracture toughness and higher brittleness. Inaddition, the higher the crack density, the higher theamount of previously detached solid particles, whichare suppressed in the microcracks and decrease thewear strength. This may result in micro-cutting, whichis considered to be the predominant wear weight loss

mechanism 16 . Hard chromium electroplates withhardness of approximately 750800 Vickers were foundto have the best frictional wear resistance, if the hard-ness was obtained as-deposited or by moderate heat

treatment of harder deposit 4 .

3.3. Salt spray tests

The results of the corrosion testing, performed in aqualitative way, were obtained by visual inspection and

by image analyser software of the specimen surfaceafter exposure to salt spray test. Both coatings com-pleted the tests fully corroded. Table 6, Figs. 14 and 15show the results of the salt spray test after 24, 48 and72 h, for all the cases.

For the HPHVOF tungsten carbide coating, a bet-ter corrosion resistance was observed after sealing theapplication before testing. However, HPHVOF tung-sten carbide coating did not quite protect the substrateagainst the aggressive action of salt spray environmentafter 72 h in the test chamber. The HPHVOF ther-mal spray tungsten carbide process has a high contentof pores and oxides, which can be detrimental towards

corrosion strength 14 . In addition, it is well knownthat the HPHVOF tungsten carbide coating does notyield consistent thickness, which also made easy thesalt spray action on sites of low thickness. On the other

Fig. 13. Abrasive wear weight loss vs. number of cycles.


Table 5Through-thickness HV microhardness with 1 N load

Microhardness-HVCoatings Surface Core Interface

Accel. hard chromium 864 920 913Conv. hard chromium 897 906 912Tungsten carbide 1070 1159 1354

hand, HPHVOF tungsten carbide coating shows thebetter performance in comparison to the hard

chromium electroplating. Bodger and co-workers 1observed no corrosion products in tungsten carbidesamples of 200 m thickness over 30 days in salt spraytest. Higher nominal thicknesses of tungsten carbidelead to discontinuous micropore distributions along thethickness, which may delay the corrosive process. Withrespect to both hard chromium plated samples, Fig. 14clearly shows the higher salt spray resistance of theaccelerated hard chromium electroplated specimenduring all tests. In relation to the accelerated hardchromium plated specimen with 49 m thickness andsubjected to 48 h in salt spray environment, it can beobserved that its surface showed approximately 5%corrosion products. On the other hand, in the samecondition, the corrosion of the conventional hardchromium electroplated specimen was full, i.e. visually,100% was corrosion. This experimental behaviour isrelated to the number of microcracks in the deposit ina way that the greater the microcrack density, the morecorrosion along the sample surface and, so, better

protection against corrosion 3 . In spite of the highermicrocrack density of the accelerated hard chromiumelectroplating, the surface roughness measurements in-dicates lower values than the conventional hardchromium electroplating, as mentioned before. In gen-eral, the corrosion resistance is related to the surfaceroughness of a part; i.e. the higher surface roughness,the higher the corrosion attack due to higher surface

area 17 . Therefore, the conventional hard chromiumelectroplating process yielded lower density and, conse-quently, deeper microcracks. It is also clear that theincrease in the thickness enhanced the hard chromium

protection to the salt spray corrosion. However, herealso, there was no protection of the substrate againstthe aggressive action of salt spray environment. Thiscorrosion is due to the high content of pores andmicrocracks inherent to the process itself, that act ascanals, leading the corrosive process to thesubstratecoating interface, and getting intensity.

4. Conclusions

The effect of tungsten carbide thermal spray coat-ing applied by HPHVOF process and hardchromium electroplating for the rotating bendingand axial fatigue tests was to decrease the fatiguestrength of AISI 4340 steel. The influence is moresignificant in high cycle fatigue tests than in lowcycle fatigue tests. The decrease of the fatiguestrength was higher in chromium electroplatedspecimens than in tungsten carbide coated speci-mens.

The higher rotating bending fatigue strength of theconventional hard chromium in comparison to theaccelerated hard chromium electroplating, despitethe higher thickness of the former, is associatedwith the lower microcrack density of the conventio-nal hard chromium electroplating.

A small increase in rotating fatigue strength wasobtained for tungsten carbide thermal spray coatedspecimens blasted with aluminium oxide in compar-ison to samples without superficial treatment.

No change in the microstructure of the base metaldue to deposition process was observed for tungstencarbide thermal spray coating applied byHPHVOF process and for chromium electroplat-ing.

For axial fatigue tests, the negative influence ofcoatings on the fatigue strength of the steel fol-lowed the same tendency observed in rotating bend-ing fatigue tests. Analysis of the hard chromiumelectroplated results revealed that the different mi-crocracks density did not play an important role intheir performance.

Table 6Results of the salt spray test in 24, 48 and 72 h

Salt spray test

Coating Tungs. carbide Accel. hard chrome Conv. hard chromeTime 100 m 16 m 36 m 49 m 16 m 36 m 49 m

24 h 30% 80% 10% OK 90% 70% 50%48 h 50% 100% 30% 5% 100% 100% 100%72 h 100% 100% 100% 100% 100% 100% 100%


Fig. 14. Salt spray test results for hard chromium electroplatedsamples after 48 h.

The wear weight loss tests showed better results forthe HPHVOF tungsten carbide coating in com-parison to the chromium electroplating. An initiallyhigher wear weight loss for the accelerated hardchromium electroplating occurred, decreasing cont-inuously with the increase in test cycles.

Both coatings completed the test of 72 h with fullcorrosion. For the HPHVOF tungsten carbidecoating, a better corrosion resistance was observedafter sealing application before testing. With re-

Fig. 15. Salt spray test results for tungsten carbide coated samplesafter 72 h.

spect to both hard chromium electroplated samples,the results indicate clearly the higher salt sprayresistance of the accelerated hard chromium elec-troplated specimens.

Acknowledgements

The authors are grateful for the support of thisresearch by CAPES, FAPESP, EMBRAER-LIEB-HERREDE and CTAAMR.

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